1. Field of the Invention
The present invention relates to a protection device for protecting an assembled battery including a plurality of secondary batteries connected in series, and a battery pack unit incorporating a protection device.
2. Description of the Related Art
A high energy density secondary battery has been widely used as a power supply for small information equipment such as a portable phone and a notebook personal computer. The secondary batteries are often connected in series, the number of which corresponds to a voltage and current necessary for an objective apparatus or often used as an assembled battery in which they are connected in series or in parallel. Because a power supply voltage of the aforementioned small information equipment is about several V to 10 V, the number of series connections in the assembled battery is 1 to 3.
On the other hand, in recent years, application of the secondary battery to purposes requiring a high output and high voltage such as household electric appliances, power tools, assisted bicycles and hybrid electric vehicles has been expanding rapidly as well as the power supply for the information equipment. Accompanied by this, the number of series connections in the assembled battery is increased, and it is not rare that 10 or more batteries are connected in series.
A prominent problem in connecting the batteries in series is a fluctuation between individual batteries (called cells). The fluctuation includes, for example, fluctuation in capacity, fluctuation in impedance, and fluctuation in the state of charge (SOC). A fluctuation which likely leads to an error of these ones is a fluctuation in voltage which is one of the fluctuations in the SOC.
If batteries having different capacities are connected in series or a plurality of batteries are connected under different SOCs, a cell having a higher voltage or a cell having a lower voltage than the average is generated in a fully charged state of the assembled battery. The cell having a higher voltage is turned into an overcharged state, whereby deterioration is intensified. If the charge is repeated, the cell whose deterioration is intensified by the overcharge has a reduced capacity, so that the overcharge is progressed, thereby accelerating the deterioration. As a result, the cycle service life of the assembled battery is shortened extremely than the service life of the cell.
The cause for the fluctuation in the SOC includes not only the fluctuation of the cell in an initial period but also a factor generated during use of the assembled battery. For example, sometime there is a difference in temperature among the cells or currents discharged from each cell are different, thereby leading to a cause for the fluctuation in the SOC. Particularly, because the protection device is complicated in the assembled battery which has many series connections to output a high voltage, the discharge current by the protection device is apt to fluctuate.
To meet such a problem, generally, a technique of eliminating a fluctuation in charging voltage by appropriately executing charge called equalizing charge is adopted for an assembled battery of a nickel hydride battery. The nickel hydride battery has a feature that if the charge is attempted to be continued further in a state near the full charge, a charge reaction of an electrode material and a decomposition/recombination reaction of water in an electrolyte turn to competing reaction, so that the charge reaction is blocked. Thus, if the charge in an area beyond the full charge is carried out under an appropriate charge condition in which no deterioration of the battery is induced, the charging voltage of each cell can be arranged equally using an electrochemical current bypass function inside the battery. The technique for the equalizing charge is described in, for example, JP-A 2001-314046 (KOKAI).
On the other hand, in a secondary battery or capacitor using a nonaqueous electrolyte, a coulomb efficiency of charge/discharge is, in general, substantially 100%. Therefore, no current bypass function can be expected inside a battery such as a nickel hydride battery. To meet such a circumstance, there has been proposed a method of providing an equalizing circuit for bypassing each cell outside an assembled battery to bypass a charged current for a cell which exceeds a predetermined voltage, thereby suppressing a fluctuation in voltage among the cells. For example, JP-A 2002-238179 (KOKAI) has disclosed a technique for connecting a zener diode to each cell of an assembled battery in parallel to bypass a charged current of the cell which exceeds a zener voltage.
Even if the technique disclosed in JP-A 2002-238179 (KOKAI) is adopted, it is difficult to effectively eliminate the fluctuation in the voltage among the cells for the reason of the following problems.
If it is attempted to suppress the fluctuation in the voltage by bypass with a single element like a zener diode, the charging voltage is controlled by the fluctuation in the zener voltage. It is technically difficult to suppress the fluctuation in the zener voltage, like to manufacture a battery having a small fluctuation.
Further, a rise-up of the zener current when the zener voltage is reached is never steep. Due to the rise-up characteristic of the zener current, a bypass current flows from a voltage lower than a necessary charging voltage. For this reason, the technique of suppressing the fluctuation in the voltage by bypass with the zener diode cannot be adopted to a secondary battery whose voltage needs to be controlled in the order of several tens mV.